1. Technical field
The present invention relates to a light emitting device capable of color-mixing light emitted from a light emitting element and light obtained through wavelength conversion of a part of the original light emitted from the light emitting element, thereby emitting light of a different color.
2. Description of Related Art
A semiconductor light emitting element such as light emitting diode is small in size, has high power efficiency and emits light with clear color. The semiconductor light emitting element also has such advantages as almost free from blowout, excellent startup performance, and high durability to vibration and repetitive operations of turning on and off. There have been developed a light emitting device capable of emitting light of various wavelengths according to the principle of color mixing of light which is obtained by combining a source light emitted from a light emitting element and a wavelength converting member which is excited by the source light and is capable of emitting light of different wavelength than that of the source light. Such light emitting devices are used as various light sources. Particularly in recent years, such light emitting devices have attracted much attention as a next-generation illumination light sources of lower power consumption and longer service life to replace fluorescent lamps, and there is increasing needs for higher light emission output and improvements in light emitting efficiency. There is also a demand for light source of higher brightness in projectors such as automobile headlight and in floodlights.
For such light emitting devices, JP 2002-141559A proposes structures as shown in FIGS. 11(a) and 11(b). The light emitting semiconductor chip assembly 72 shown in FIG. 11(a) has a light emitting diode chip 74 fixed on a fluorescent material chip 74 through a transparent adhesive material 76. The fluorescent material chip 74 has a fluorescent material layer 82 on a base member 80 made of a transparent material such as silica or alumina or an opaque material having high light reflectivity. FIG. 11(b) shows a cross-sectional view of a light emitting device 92 constituted by using the light emitting semiconductor chip assembly 72. The light emitting device 92 has an anode lead 88 and a cathode lead 90, and the light emitting semiconductor chip assembly 72 is fixed in the cap portion 90a provided at an end of the cathode lead 90. An anode electrode 84 and a cathode electrode 86 of the light emitting semiconductor chip assembly 72 are respectively connected to the anode lead 88 and the cathode lead 90. The surroundings of the light emitting semiconductor chip assembly 72 is enclosed with a protective adhesive material 96 having a light scattering agent 94 dispersed therein.
The light emitting device shown in FIGS. 11(a) and 11(b) has the phosphor material chip 74 fixed on the back surface of the light emitting diode chip 78, which enables an increase in optical output compared to a light emitting device in which the back surface of the light emitting diode chip 78 is directly adhered in the cup portion 90a of the cathode lead. The reason for this is considered as below. In the case where the back surface of the light emitting diode chip 78 is directly adhered in the cup portion 90a of the cathode lead by using a silver paste and the like, light emitted from the back surface of the light emitting diode chip 78 is reflected by the silver paste. However, the reflectance of a silver paste is not sufficiently high and, in addition, a large proportion of the reflected light returns to the light emitting diode chip 8 and is absorbed, resulting in a decrease in the optical output. Adhering the fluorescent chip 74 on the back surface of the light emitting diode chip 78 decreases the ratio of the light emitted from the back surface of the light emitting diode chip 78 returning in the light emitting diode chip 78, so that the light can be efficiently extracted outside through the fluorescent material layer 82, and thus the optical output can be increased. Due to the effect of the light scattering agent 94 dispersed in the protective adhesive material 96, the colors of light emitted from the light emitting diode chip 78 and the fluorescent material chip 74 are mixed, so that color unevenness of emitted light can be suppressed.
However, with such a conventional light emitting device, sufficient improvement in both color unevenness and light emission output has been difficult to obtain. That is, to sufficiently reduce the color unevenness by mixing the colors of light emitted from the light emitting diode chip 78 and the fluorescent material chip 74, to some extent a large amount of the light scattering agent 94 is needed to be dispersed. However, if a large amount of the light scattering agent 94 is dispersed around the light emitting diode chip 78, the amount of light returning in the light emitting diode chip 78 after being reflected by the light scattering agent 94 increases, resulting in an increase in the ratio of light absorbed in the light emitting diode chip 78. Thus, if the amount of the light scattering agent 94 is increased to improve the color unevenness, then the light emission output decreases, and if the amount of the light scattering agent 94 is decreased to enhance the light emission output, then the color unevenness increases. Accordingly, an object of the present invention is to provide a novel light emitting device that can improve both the color unevenness and light emission output of the light emitting device at the same time.